[RE-wrenches] combiners and the 120% rule

[Kirk Herander]

Jason,

 

In your email below you state:

 

"You DO need to observe the 120% rule for the combining subpanel, regardless
of whether there are loads present, at least in jurisdictions where I have
worked. I've heard that some inspectors will allow you to ignore it if it is
labeled as a PV combiner with "add no loads" notation."

 

NEC 705.12 (D) states that the distribution equipment (in this case the
combiner panel, fed by multiple inverters and a utility source) must be
"capable of supplying multiple branch circuits or feeders or both" for
(D)(1) through (7) to apply. If you fully populated a combiner panel with
inverter breakers, leaving no slots for load breakers, it is not capable of
supplying branch circuits or feeders, and IMO the 120% rule does not apply
to the combiner buss or the conductors back to its point of utility
interconnect. I have argued this point as well as label combiners "load
circuits prohibited" (with or without available slots) and received AHJ
approval.

 

You could also just lock shut a combiner that had spare slots as a deterrent
to adding load breakers.

 

 

Kirk Herander

VT Solar, LLC

dba Vermont Solar Engineering

NABCEPTM Certified installer Charter Member

NYSERDA-eligible Installer

VT RE Incentive Program Partner

802.863.1202

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jason
Szumlanski
Sent: Tuesday, June 26, 2012 8:28 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] Enphase grid tie question

 

I'll email you off-list a 1-line diagram from a system with 164
microinverters broken down into 8 strings in a 208V system. This particular
system used two subpanels to accumulate PV, but that was only because we had
to backfeed two existing subpanels due to the size of existing 480/208V
transformers. You will have to look at the utility service and all existing
equipment.

 

Regarding the breakers in the subpanel, you will only need a maximum of a
20A breaker for each string. The max inverters per string is 25 and the
calculation for OCPD is:

 

215W / 208V x 25 inverters / 1.732 x 1.25 = 18.65A

 

"You DO need to observe the 120% rule for the combining subpanel, regardless
of whether there are loads present, at least in jurisdictions where I have
worked. I've heard that some inspectors will allow you to ignore it if it is
labeled as a PV combiner with "add no loads" notation."

 

Use a MLO panel with a fusible disconnect between the subpanel and the
interconnection point. If you use a 225A panel, you can feed it with 270A.
With eight 20A backfed PV circuits, you would need to protect the line side
of the panel with a 100A fusible disconnect. That probably isn't going to
work. You may be best off from a cost perspective using two 225A subpanels
and two 60A fusible disconnects. Anything larger than a 60A 3P disconnect
and the price skyrockets. It all depends on your circuit calculations and
the existing equipment. Of course, you would need two spaces for your
interconnection point.

 

Jason Szumlanski

Fafco Solar

 

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